The present invention relates to a voltage-driven type semiconductor device, and more particularly to a voltage-driven type semiconductor device suitable to be used for a power converter and a power source of various power capacities, a power amplifier, an oscillator, an analog switch or the like as a single device or by integrating a plurality of such devices into an IC.
As voltage-driven type semiconductor devices of this sort according to prior art techniques, a MOSFET, an SI transistor, an IGBT, a MOS thyristor and so on are known. Being driven by voltage, these semiconductor devices have such a feature that the power dissipation for driving is very small as compared with a current-driven type semiconductor device. Further, MOSFETs and SI transistors have an advantage that an on-current can be applied even in case of low on-voltage in the vicinity of 0 V. On the other hand, in such devices it is required to relieve an electric field intensity by expanding a depletion layer into a drain region and also to lower impurity concentration in the drain region in order to use the devices at a high voltage.
Thus, in a high voltage MOSFET and a high voltage SI transistor, resistance of a drain region becomes high. As a result, the on-resistance of the whole element becomes high, and the power dissipation of the element becomes very large.
A different type of voltage-driven device, known as an IGBT, is described in, for instance, JP-A-4-11780.
The IGBT according to this prior art arrangement is a voltage-driven type semiconductor device in which an emitter for carrier injection is connected to a drain of a MOSFET. Since carriers (such as holes) are injected into the drain in the on-state of the element and conductivity modulation is produced, the IGBT has such an advantage that the on-resistance thereof can be reduced to approximately 1/4 of that of a MOSFET even when it is designed for a high voltage. A consequence, it is possible to reduce the power dissipation of the device by a large margin.
The IGBT which is a voltage-driven type semi-conductor device, however, is latched up when carriers (such as electrons) having a different polarity are injected from the source thereof. Therefore, it is of vital importance to check injection of the carriers having a different polarity, thereby to prevent voltage control from becoming inexecutable by latch-up.
Another prior art technique is described in, for example, IEEE Trans. Electron Devices, Vol. ED-33, pp. 1609-1618 (1986) written by V.A.K. Temple, JP-A-3-87068, and is known as a prior art arrangement related to a MOS thyristor which is a voltage-driven type semiconductor device.
A MOS thyristor according to this prior art arrangement has basically a pnpn structure similar to that of an IGBT, in which carriers (such as electrons) are injected positively also from an emitter corresponding to a source of the IGBT and conductivity modulation in a base region corresponding to a drain of the IGBT is made deeper so as to generate latch-up, thereby to further reduce the on-resistance even more than an IGBT. Thus, it is possible to reduce the power dissipation in this MOS thyristor lower than the IGBT described above by a large margin.
However, it has been required in such prior art MOS thyristors to include inside the MOSFET an arrangement for short-circuiting the emitter junction in order to make it possible to control the off-voltage after latch-up.
Since the on-resistance can be reduced remarkably in the IGBT and the MOS thyristor according to a prior art as compared with a MOSFET as described above, they are suitable to be used for high voltage and large current. In these elements, however, it is impossible to apply a current from the vicinity of 0 V, and voltage drop caused by the junction at about 0.6 to 0.8 V exists. Therefore, power dissipation by the voltage drop portion is produced even if conductivity modulation is made sufficiently deep, thus limiting reduction in power dissipation of the whole element. Namely, a linear characteristic has been unobtainable in volt-ampere characteristics of these elements.
The voltage-driven type semiconductor device such as an IGBT and a MOS thyristor according to a prior art described above has a problem that reduction of power dissipation of the whole element is limited since voltage drop caused by the junction at about 0.6 to 0.8 V exists and power dissipation caused by the voltage drop portion is produced even when conductivity modulation is made sufficiently deep.